Re: [PATCH qemu] spapr: Use address from elf parser for kernel address

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 5/18/22 04:58, Daniel Henrique Barboza wrote:
> Alexey,
> 
> I had to amend your commit due to Gitlab CI complaining about ...
> 
> On 5/4/22 03:55, Alexey Kardashevskiy wrote:
>> tl;dr: This allows Big Endian zImage booting via -kernel + x-vof=on.
>>
>> QEMU loads the kernel at 0x400000 by default which works most of
>> the time as Linux kernels are relocatable, 64bit and compiled with "-pie"
>> (position independent code). This works for a little endian zImage too.
>>
>> However a big endian zImage is compiled without -pie, is 32bit, linked to
>> 0x4000000 so current QEMU ends up loading it at
>> 0x4400000 but keeps spapr->kernel_addr unchanged so booting fails.
>>
>> This uses the kernel address returned from load_elf().
>> If the default kernel_addr is used, there is no change in behavior (as
>> translate_kernel_address() takes care of this), which is:
>> LE/BE vmlinux and LE zImage boot, BE zImage does not.
>> If the VM created with "-machine kernel-addr=0,x-vof=on", then QEMU
>> prints a warning and BE zImage boots.
>>
>> Note #1: SLOF (x-vof=off) still cannot boot a big endian zImage as
>> SLOF enables MSR_SF for everything loaded by QEMU and this leads to early
>> crash of 32bit zImage.
>>
>> Note #2: BE/LE vmlinux images set MSR_SF in early boot so these just 
>> work;
>> a LE zImage restores MSR_SF after every CI call and we are lucky enough
>> not to crash before the first CI call.
>>
>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>> ---
>>
>> We could probably change SLOF to always clear MSR_SF before jumping to
>> the kernel but this is 1) SLOF fix 2) not quite sure if it brings
>> lots of value.
>>
>>
>> I really wish I had this when tested this fix:
>> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20220406070038.3704604-1-aik@ozlabs.ru/
>>
>> ---
>>   hw/ppc/spapr.c | 12 ++++++++++--
>>   1 file changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
>> index a4372ba1891e..89f18f6564bd 100644
>> --- a/hw/ppc/spapr.c
>> +++ b/hw/ppc/spapr.c
>> @@ -2971,14 +2971,16 @@ static void spapr_machine_init(MachineState 
>> *machine)
>>       }
>>       if (kernel_filename) {
>> +        uint64_t loaded_addr = 0;
>> +
>>           spapr->kernel_size = load_elf(kernel_filename, NULL,
>>                                         translate_kernel_address, spapr,
>> -                                      NULL, NULL, NULL, NULL, 1,
>> +                                      NULL, &loaded_addr, NULL, NULL, 1,
>>                                         PPC_ELF_MACHINE, 0, 0);
>>           if (spapr->kernel_size == ELF_LOAD_WRONG_ENDIAN) {
>>               spapr->kernel_size = load_elf(kernel_filename, NULL,
>>                                             translate_kernel_address, 
>> spapr,
>> -                                          NULL, NULL, NULL, NULL, 0,
>> +                                          NULL, &loaded_addr, NULL, 
>> NULL, 0,
>>                                             PPC_ELF_MACHINE, 0, 0);
>>               spapr->kernel_le = spapr->kernel_size > 0;
>>           }
>> @@ -2988,6 +2990,12 @@ static void spapr_machine_init(MachineState 
>> *machine)
>>               exit(1);
>>           }
>> +        if (spapr->kernel_addr != loaded_addr) {
>> +            warn_report("spapr: kernel_addr changed from 0x%lx to 
>> 0x%lx",
>> +                        spapr->kernel_addr, loaded_addr);
> 
> 
> ... this code. This is problematic when compiling in a 32 bit 
> environment because
> the definition of long (long) unsigned differs from the usual 64 bit env 
> we use:
> 
> 
> 
> ../hw/ppc/spapr.c: In function 'spapr_machine_init':
> ../hw/ppc/spapr.c:2998:25: error: format '%lx' expects argument of type 
> 'long unsigned int', but argument 2 has type 'uint64_t' {aka 'long long 
> unsigned int'} [-Werror=format=]
>   2998 |             warn_report("spapr: kernel_addr changed from 0x%lx 
> to 0x%lx",
>        |                         
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   2999 |                         spapr->kernel_addr, loaded_addr);
>        |                         ~~~~~~~~~~~~~~~~~~
>        |                              |
>        |                              uint64_t {aka long long unsigned int}
> ../hw/ppc/spapr.c:2998:25: error: format '%lx' expects argument of type 
> 'long unsigned int', but argument 3 has type 'uint64_t' {aka 'long long 
> unsigned int'} [-Werror=format=]
>   2998 |             warn_report("spapr: kernel_addr changed from 0x%lx 
> to 0x%lx",
>        |                         
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   2999 |                         spapr->kernel_addr, loaded_addr);
>        |                                             ~~~~~~~~~~~
>        |                                             |
>        |                                             uint64_t {aka long 
> long unsigned int}
> cc1: all warnings being treated as errors
> 
> 
> I've fixed it by doing the following:
> 
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 156e799ae9..8d5bdfc20f 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -2995,7 +2995,7 @@ static void spapr_machine_init(MachineState *machine)
>           }
> 
>           if (spapr->kernel_addr != loaded_addr) {
> -            warn_report("spapr: kernel_addr changed from 0x%lx to 0x%lx",
> +            warn_report("spapr: kernel_addr changed from 0x%"PRIx64
> +                        " to 0x%"PRIx64,
>                           spapr->kernel_addr, loaded_addr);
>               spapr->kernel_addr = loaded_addr;
>           }
> 
> 
> 
> If you're ok with this fixup we can keep it as is. Otherwise feel free 
> to send
> another version.


I am totally fine with this change, sorry I have not compile tested it, 
just assumed this cannot fail :-/


> 
> 
> Thanks,
> 
> 
> Daniel
> 
> 
> 
> 
> 
>> +            spapr->kernel_addr = loaded_addr;
>> +        }
>> +
>>           /* load initrd */
>>           if (initrd_filename) {
>>               /* Try to locate the initrd in the gap between the kernel

-- 
Alexey